What is the difference between NPSHa and NPSHr?

NPSHa is calculated from the suction system. NPSHr is supplied by the pump manufacturer for a particular flow and speed.

Can static head be negative?

Yes. Enter a negative value when the pump centreline is above the liquid surface, creating suction lift.

Should velocity head be included?

The equation shown uses total head at the pump suction. Be consistent with the manufacturer definition and the location where losses are evaluated.

Does positive margin guarantee no cavitation?

No. NPSH margin is important, but cavitation and recirculation behaviour also depend on pump design and operating point.

Drawing No. EH–FM–021 // Fluid Mechanics & Piping

Pump NPSH Calculator

Reviewed August 2026

Calculate NPSH available and margin for an open or pressurized suction source, including water vapour pressure from temperature.

Scope: Preliminary hydraulic screening. Use the pump manufacturer’s NPSHr curve and the project’s NPSH margin policy for final selection.

What problem does this solve?

Calculate net positive suction head available (NPSHa) and compare it with the pump manufacturer's NPSHr.

1. Set liquidTemperature determines vapour pressure.

2. Set pressureAtmospheric or vessel pressure acts on the liquid surface.

3. Describe suctionStatic level, pipe loss and inlet velocity reduce or increase head.

4. Compare marginNPSHa must exceed NPSHr with an appropriate project margin.

Inputs

Liquid temperature °C

Liquid density kg/m³

Gravity m/s²

Pressure on liquid surface kPa absolute

Vapour pressure kPa absolute

Water vapour pressure is updated from temperature. Override for another liquid.

Static liquid level above pump m

Suction friction loss m

Suction velocity m/s

Pump NPSHr m

Desired extra margin m

Results

Surface pressure head

—

Vapour pressure head

—

Velocity head

—

NPSH available

—

NPSH margin

—

Screening status

—

Important: NPSHr is normally determined by the pump manufacturer at a specified head-drop criterion. Cavitation risk also depends on suction energy, recirculation, air entrainment, operating point, fluid properties and margin policy.

Background

Engineering basis, assumptions, interpretation and practical limitations.

Frequently Asked Questions

Practical questions about NPSHa, NPSHr, suction head and cavitation margin.